1. Field of the Invention
The present invention relates to a reinforcing carbon fiber for use in a carbon-carbon composite material obtained by impregnating an aggregation of a carbon as the reinforcing fiber with a liquid carbonizable substance as the precursor of matrix carbon and carbonizing the resulting material, followed by graphitization thereof if necessary. More particularly, the present invention relates to a reinforcing carbon fiber for use in a high-strength carbon-carbon composite material excellent in heat resistance, chemical resistance, etc.
2. Related Art
The interfacial adhesion between the reinforcing carbon fiber and matrix carbon of a carbon-carbon composite material is very poor as compared with those of other composite materials such as carbon fiber-reinforced plastics (CFRP).
A carbon-carbon composite material is produced according to the following procedure. Either a high-strength, high-elasticity carbon fiber still being wound up into a desired morphology, or a carbon fiber structure mainly constituted of a high-strength, high-elasticity carbon fiber to take the form of a woven fabric, a three-dimensional fabric, a non-woven fabric, a unidirectional sheet or the like is impregnated with a thermosetting resin such as a phenolic resin or a furan resin as a precursor of matrix carbon, followed by shaping and curing thereof to form a preliminary carbon-plastic composite material.
The carbon-plastic composite material is carbonized through a heat treatment under an inert atmosphere to obtain a carbonized product in the form of a carbon-carbon composite material or a skelton of a carbon-carbon composite material. If necessary, the densification, or secondary reinforcement, treatment procedure of further impregnating the carbonized product with a thermosetting resin, pitch or the like as the precursor of matrix carbon and subsequently carbonizing the impregnated product is repeated to secure desired properties of the resulting carbon-carbon composite material.
Whether such a secondary reinforcement treatment is necessary or not may be determined together with the number of times of the secondary reinforcement treatment, if necessary, according to the end use application of the final carbon-carbon composite material and the like. In the classical case of using the carbon-carbon composite material as a heat-resistant material or the like, the secondary reinforcement treatment is hardly necessary.
Since a significantly high level of strength has recently been required of carbon-carbon composite materials in a widening variety of fields of applications thereof in addition to the application thereof as heat-resistant materials, however, the necessity of the secondary reinforcement treatment has been increasing accordingly.
The significance of the secondary reinforcement treatment using the precursor of matrix carbon lies in an improvement in the strength of the carbon-carbon composite material by filling carbon into defects ensuing from poor interfacial adhesion between the reinforcing carbon fiber and matrix carbon of the carbon-carbon composite material obtained in the early stage of carbonization, more specifically into separations, cracks or the like along the interfaces between the above-mentioned reinforcing carbon fiber and matrix carbon.
In the case of using a thermosetting resin as the precursor of matrix carbon, the matrix carbon after carbonization is sometimes observed as being separated from the surfaces of the reinforcing carbon fiber to form defects which are usually referred to as separations.
On the other hand, where a pitch matrix type of carbon fiber-reinforced composite material formed through impregnation an aggregation of carbon fiber with pitch is carbonized to produce a carbon-carbon composite material, matrix pitch carbon existing between carbon fibers, though stuck to the surfaces of the fibers, includes therein separation defects formed along the closest extending carbon fiber to the middle of the area of matrix carbon, or crescent defects like something reflective of the velocity gradient profile of a fluid in a state of laminar flow, with the result that the composite material is observed as being poor in the interfacial adhesion between the carbon fiber and the matrix. Such defects are usually referred to as cracks.
A carbon-carbon composite material having such defects as separations or cracks can be improved in physical properties such as mechanical strength in particular by repeating the secondary reinforcement treatment procedure to densify the composite material. Since the secondary reinforcement treatment can only decrease the proportion of existing defects, however, the defects present along the extending, reinforcing carbon fiber resisting stress put on the carbon-carbon composite material cannot essentially be obviated.
Pitch which may be either of coal origin or of petroleum origin is predominantly used as the precursor material of matrix carbon for the secondary reinforcement treatment therewith in an economical aspect and from the viewpoint of the yield of carbon through carbonization.
Pitch forms a mesophase exhibiting extreme anisotropy in terms of optical texture in the course of heat treatment thereof. The texture of carbon formed from pitch also shows extreme anisotropy like graphitic material.
Consequently, the secondary reinforcing material formed through the above-mentioned secondary reinforcement treatment to fill up the neighborhoods of the reinforcing carbon fibers provides carbon of graphitic texture showing optical anisotropy, and, hence, includes therein laminar defects innate as in most of graphitic carbon materials. This entails a problem yet to be solved in improving the shear strength of the carbon-carbon composite material though the above-mentioned matrix pitch carbon can fulfill the role of carbonaceous filling material.
In order to improve the interfacial adhesion of a reinforcing carbon fiber to the matrix carbon of a carbon-carbon composite material, it is a common practice to use a surface-treated type of reinforcing carbon fiber having the surface thereof subjected to an oxidation treatment such as an electrolytic oxidation treatment to introduce thereinto functional groups in a similar way to that in the case of production of plastic composite materials.
This can not only definitely improve the adhesion of the reinforcing carbon fiber to the precursor of matrix carbon but also increase the interfacial adhesion in some local sites between the reinforcing carbon fiber and the matrix carbon even after carbonization, but this localized, increased interfacial adhesion is so strong that separations and cracks are caused in the other weakly adherent interfacial sites in keeping with great shrinkage of the precursor of matrix carbon during carbonization.
When an object of the carbon-carbon composite material is small in size, the apparently small amount of shrinkage of the object during carbonization can keep the morphology of the object intact. On the other hand, when an object of the carbon-carbon composite material is large in size, defects are quite often formed during carbonization. Particularly when the object is a laminate of carbon fiber fabrics, fatal interfacial delamination and cracking are brought about during the course of carbonization. Thus, the use of the surface-treated type of reinforcing carbon fiber cannot be said to be preferable.
Japanese Patent Laid-Open No. 52,912/1977 discloses the use of the same kind of precursor of matrix carbon as the starting material of reinforcing carbon fiber, in which case the matrix carbon formed through carbonization of the precursor thereof shows substantially the same properties as the reinforcing carbon fiber. This enables a difference therebetween in thermal expansion coefficient to be minimized, with the result that a heat treatment, if necessary, in a high-temperature range can be effected with a decrease in defects such as cracks and separations formed around the interfaces between the reinforcing carbon fiber and the matrix carbon. In this sense, the foregoing technology is effective to some extent.
Since the shrinkage of the precursor of matrix carbon is large in the early phase of the carbonization, however, defects are liable to be formed around interfaces between the reinforcing carbon fiber and the matrix carbon being formed. In this sense, that technology still involves a problem yet to be solved.
Japanese Patent Laid-Open Nos. 127,264/1985 and 127,265/1985 disclose the use of carbon fibers having the surfaces thereof coated with a phenolic resin or a pitch-modified phenolic resin by kneading the two materials together, as the reinforcing material of a carbon-carbon composite material.
The disclosed technologies are effective in production of a short fiber-reinforced type of carbon-carbon composite materials, but generally inapplicable to production of a filament-reinforced type of high-strength carbon-carbon composite materials. Furthermore, since the coating material is also bound to serve as the precursor of matrix carbon, however, the problem with the difference in shrinkage between the reinforcing carbon fibers and the matrix carbon in the early phase of the carbonization, which difference is particularly problematic in the interfacial portions of both materials, is yet to be solved.
The use of a sizing or coupling agent such as a silane compound or an epoxy compound, which is commonly applied to glass fibers and the like as reinforcing materials generally for reinforced plastics, may be conceived of with the aim of improving the interfacial adhesion between the reinforcing carbon fiber and matrix carbon of a carbon-carbon composite material. However, this is not desirable because the use of an epoxy sizing agent does not improve the interfacial adhesion between the reinforcing carbon fiber and matrix carbon of a final composite material because of its poor affinity for the precursor of matrix carbon and of its low yield of carbon by carbonization, while the use of a coupling agent such as a silane compound, if still present in the final heat treatment step, causes a decrease in the strength of the resulting carbon-carbon composite material, leading to a necessity of removing the coupling agent before the final heat treatment step.